[Elecraft] CM500 Powering from K2

[Jim Brown]

A week or so ago, KS4L asked for info on powering the CM500 from 
his K2. He tried the instructions I gave, but it didn't work. Bear 
in mind here that several variations of output stages are used for 
the FET impedance converter in electret mics, and not all mfrs wire 
them the same way to their plugs. For this reason, a single 
powering method doesn't work for all electret mics. We learned this 
two decades ago in the pro audio world when we started using 
lavalier (clip-on) mics with wireless mic transmitters, and every 
mfr of both mics and wireless systems published lots of info about 
how to hook their stuff up. 

The CM500 works directly with the K3, so I looked at what the K3 
does for power. It's 5.6K to the tip only. The K3 makes no 
connection to the ring. Randy tried that and it works fine. 

So -- revised advice. From the CM500 mic plug, wire the tip to the 
mic input, add a 5.6K resistor between +DC on the K2 mic jack and 
the mic input. The resistor value is not critical, 4.7K to 8.2K 
should work fine. There is very little current, so very small 
resistors are fine. The sleeve (common) of the plug goes to common 
at the mic jack. 

The same wiring should work with this mic for nearly all other ham 
rigs. 

I would expect the CM500 to work really well with the K2. One of 
the few shortcomings of the K2 is that since it was originally 
designed as a QRP CW rig, the support for SSB is not a strong 
point. One way this shows up is in relatively low audio gain and 
not enough LF rolloff. The CM500 helps this situation, first 
because it's a pretty "hot" mic (that is, higher than average 
output level) and because the low end is rolled off internally. 

There are several published mods to improve the gain of the SSB 
signal chain, some of which go all the way to the RF end of the 
chain. I took the simpler approach, modifying my K2s by changing a 
few resistors and capacitors in the mic stages to provide rolloff 
around 500 Hz and increase the gain by about 7dB. This allows the 
mic to hit the compressor harder (and the rolloff prevents low 
frequency sounds from hitting the compressor), making the audio 
more competitive. The CM500 should provide the same improvements 
but without the mods. 

73,

Jim Brown K9YC